Recently, we reported the discovery of a new reaction in which a cyclic anhydride randomly inserts into carbon-oxygen bonds of a polyether to generate a polymeric composition having both ether and ester functionalities (see Appl. Ser. No. 07/979,760, now U.S. Pat. No. 5,319,006). A Lewis acid such as zinc chloride or zinc bromide catalyzes the reaction.
When a polyether polyol reacts with a cyclic, saturated anhydride, for example, the product is a saturated polyetherester polyol useful for polyurethane applications. Cyclic, unsaturated anhydrides such as maleic anhydride can be used in the process to make unsaturated polyetherester resins. The unsaturated resins can be reacted with vinyl monomers to produce cured polyetherester products.
Compared with the synthesis of conventional unsaturated polyester resins, the process for making polyetheresters by insertion of an anhydride has great flexibility. The average polyether chain length between ester linkages and the crosslinkability of the polyetherester are controlled by simply adjusting the proportion of cyclic, unsaturated anhydride used. Products having a wide range of unsaturation levels are available from a single polyether polyol and a single cyclic, unsaturated anhydride.
We also applied the Lewis acid-catalyzed insertion process to the reaction of polyethers and acyclic anhydrides to make glycol diesters (U.S. Pat. No. 5,254,723). Using this process, a relatively crude polyether polyol mixture can be converted with acetic anhydride to a mixture of glycol diacetates. The glycol diacetates are easily purified by distillation, and can be used as solvents or chemical intermediates.
The Lewis acid-catalyzed process for anhydride insertion has some drawbacks. For example, the activity of the catalysts is somewhat lower than desirable. Typically, at least about 1 wt. % of the Lewis acid catalyst is needed for good activity in making the polyetherester. Second, the polyetherester products often have a higher degree of color than is desirable. Third, the presence of high levels of residual Lewis acid catalysts in the polyetherester product can have an unfavorable impact on performance in various end uses. Fourth, a significant amount of volatile by-products are generated in making the polyetheresters. In addition, Lewis acids are often not satisfactory for use in manufacturing operations because they tend to attack reactors and other processing equipment.
A key limitation of the Lewis acid-catalyzed insertion process for making polyetheresters is that the reaction does not appear to work for carboxylic acids. As a practical matter, the cost and availability of various cyclic anhydrides limit the kinds of polyetherester products that can be made.
Because of the wide range of available dicarboxylic acids, and the relatively low cost of most dicarboxylic acids relative to the corresponding anhydrides, a process that would enable insertion of dicarboxylic acids into polyethers to give polyetheresters would be valuable. A preferred process could use aliphatic and aromatic dicarboxylic acids commonly used for making polyester resins, such as isophthalic acid, adipic acid, and the like. A preferred process would avoid some of the other disadvantages of the Lewis acid-catalyzed process for making polyetheresters by anhydride insertion, such as the generation of volatile by-products. Ideally, the process would give low-color polyetheresters useful for a variety of products, including unsaturated polyesters and polyurethanes.